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This article first appeared in the PB March 2006 issue of Pro Builder.

Much news has been made in recent years of mold and other toxic substances getting into homes and making residents sick. As a result, home builders need to be keenly aware of how the construction process can affect the indoor air quality in the homes they build. If builders use good construction techniques, pay attention to detail and plan ahead, they are more likely to build homes where the air is healthy, safe and comfortable for its inhabitants.

According to the EPA's "The Inside Story: A Guide to Indoor Air Quality," people spend approximately 90 percent of their time indoors. Scientific evidence indicates that air quality within homes can be worse than outside air quality in even the most polluted metropolitan areas. So homes with poor indoor air quality pose a serious healthy risk for many people.

The primary causes of poor indoor air quality are pollutants that infiltrate the home from the outside and those that emanate from materials within. Poor ventilation compounds the problem by trapping bad air inside and keeping fresh air out.

Builders can anticipate possible compromises to indoor air quality and be proactive in preventing them.

"The first thing [builders] need to do is control the sources of pollution," says Asa Foss, a building science expert with PATH.

Builders need to make sure they are constructing a tight building envelope. Outdoor air flows into homes through openings and cracks in walls, floors, and ceilings, and around windows and doors. Improperly sealed ductwork will also cause air leaks.

"Builders have gotten very good at chasing those leaks ... and energy losses during the construction process," says Dana Bres, a research engineer at PATH.

Two major areas where contaminants get into the home are through attics and crawlspaces.

"Those two areas have very large amounts of dust and very uncomfortable temperature and humidity levels," says Foss. "They often have high levels of fecal matter from rats and insects and things like that. They're really not healthy spaces to be in. Avoiding any airflow or air exchange between those two places is an absolute must."

Other areas of concern are combustion closets where water heaters, furnaces and other gas-fueled appliances are often housed, and attached garages. If airflow is allowed between them and the indoor living space, unhealthy amounts of carbon monoxide can seep into the home.

"Make sure you have an effective air barrier between the garage and adjacent spaces so there's no exchange of air," says Stan Gatland, manager, Building Science.

"You want to maintain a neutral pressure between the two spaces, or if the pressure is slightly positive, you'll want to make sure the exchange is from the inside to the outside, not from the outside in."

"You have to design your home so that you're not pulling combustion products back into the house," says Gatland.

When several types of combustible equipment are running simultaneously in a home with insufficient ventilation, there may not be enough ventilation outlets to allow all of the appliances to exhaust freely, according to Gatland.

"In this scenario of competing fans," Gatland says, "the strongest fan wins. The equipment with the weakest fan can cause a backdraft of combustion products like carbon monoxide or smoke to escape into the house."

A combustion closet should have a vent that leads to the outside. To avoid backdrafts, builders should install appliances that use direct vent combustion.

Materials used in the building of homes — paints, finishes, carpets, cabinetry or other furniture made of certain pressed wood products — have chemical compounds that can vaporize into the air. These are called volatile organic chemicals, or VOCs.

"Essentially every product in a home off gasses a certain amount of VOCs," says Foss. "Some do it much more than others."

But many of these products are now manufactured with low or no VOC options.

"They all will cost a little bit more," says Foss," but they are very easy substitutes.

An important and well-known indoor air contaminant is mold.

"To control mold, you have to control moisture," says Foss. "Most builders know this and they do it reasonably well. They pay a little special extra attention to making sure that there's a good drainage plane on the exterior walls.

"Building overhangs would be very beneficial, because they keep the water from hitting the walls and possibly getting moisture into the home. ... Make sure that gutters and downspouts keep water at least three feet away from the home."

Foss suggests that builders might also want to replace mold resistant gypsum for drywall on bathroom walls.

Too Much of a Good Thing

The fact that today's homes are more air tight than ever can cause problems. If a house does not get enough fresh air, pollutants can build up to unhealthy levels.

"If the home is too tight, then what you need to do is induce mechanical ventilation," says Bres.

The EPA, in the publication referenced on page 103, says a range of ventilators can be used — anything from outdoor-vented fans that intermittently remove air from a single room, to air handling systems that use fans and duct work to continuously remove indoor air and distribute filtered and conditioned outdoor air to strategic points throughout the house.

Installing a mechanical ventilation system at the design and construction phase is not particularly expensive.

"As a retrofit, it gets a whole lot worse," says Bres. "Obviously coming back and fixing something later is always more expensive than doing it during construction."

Consult With the Experts

"Builders should work with their heating and ventilating contractors and ask questions," says Bres. "Don't accept the fact that we're doing it this way because this is the way we did it last year. Ask questions. How can we make the system better?"

Planning ahead is the most cost efficient and effective way to insure indoor air quality.

"That's the key to it all," says Foss. "If you think of things first, they're going to be very easy things [to implement]. And there are going to be improvements not only in the indoor air quality, but in the durability of the home and the energy efficiency."